Understanding the PSTN

All the signaling standards and communication methods discussed in the previous section typically focus around the connection to one, massive voice network known as the public switched telephone network (PSTN). If you have ever made a call from a home telephone, you have experienced the results of the traditional telephony network. This network is not unlike many of the data networks of today. Its primary purpose is to establish worldwide pathways allowing people to easily connect, converse, and disconnect.

The Pieces of the PSTN

When the phone system was originally created, individual phones were wired together to allow people to communicate. If you wanted to connect with more than one person, you would need multiple phones. As you can imagine, this solution was short lived as a more scalable system was found. The modern PSTN is now a worldwide network (much like the Internet) built from the following pieces, as shown in Figure 1.13:

■ Analog telephone: Able to connect directly to the PSTN and is the most common device on the PSTN. Converts audio into electrical signals.

■ Local loop: The link between the customer premises (such as a home or business) and the telecommunications service provider.

■ Central office (CO) switch: Provides services to the devices on the local loop. These services include signaling, digit collection, call routing, setup, and teardown.

■ Trunk: Provides a connection between switches. These switches could be CO or private.

■ Private switch: Allows a business to operate a "miniature PSTN" inside its company. This provides efficiency and cost savings because each phone in the company does not require a direct connection to the CO switch.

■ Digital telephone: Typically connects to a PBX system. Converts audio into binary 1s and 0s, which allows more efficient communication than analog.

Figure 1.13 PSTN Components

Many believe the PSTN will eventually be absorbed into the Internet. While this may be true, advances must be made on the Internet to ensure proper quality of service (QoS) guarantees for voice calls.

Understanding PBX and Key Systems

Many businesses have hundreds or even thousands of phones they support in the organization. If the company purchased a direct PSTN connection for each one of these phones, the cost would be astronomical. Instead, most organizations choose to use a private branch exchange (PBX) or key system internally to manage in-house phones. These systems allow internal users to make phone calls inside the office without using any PSTN resources. Calls to the PSTN will forward out the company's PSTN trunk link (refer to Figure 1.13).

When you first look at a PBX system, it looks like a large box full of cards. Each one of these cards has a specific function:

■ Line cards: Provide the connection between telephone handsets and the PBX system.

■ Trunk cards: Provide connections from the PBX system to the PSTN or other PBX systems.

■ Control complex: Provides the intelligence behind the PBX system; all call setup, routing, and management functions are contained in the control complex.

If you look at a PBX from a network equipment mindset, "single point of failure" might be one of the first thoughts that jump into your mind. While this may be true, most PBX systems offer 99.999 percent uptime with a lifespan of 7 to 10 years. That's a hard statistic to beat in just about any industry.

Key systems are geared around small business environments (typically less than 50 users). As technology has advanced, the line between key systems and PBXs has begun to blur; however, key systems typically support fewer features and have a "shared line" feel. For example, you might see a key system installed in a small insurance office where users all have four lines assigned to their phone. If Joe were to use line 1, the line would appear busy for all users at the insurance office.

Note Although key systems often have a shared-line feature set, many key systems have numerous features that allow them to operate just like a PBX system, but with fewer ports.

Connections to and Between the PSTN

When you want to connect to the PSTN, you have a variety of options. Home users and small offices can connect using analog ports. Each two-wire analog connection has the ability to support a single call. For home users, a single, analog connection to the PSTN may be sufficient. For small offices, the number of incoming analog connections directly relates to the office size and average call volume. As businesses grow, you can consolidate the multiple analog connections into one or more digital T1 or E1 connections, as shown in Figure 1.14.

Pstn Connections

Large Office

Figure 1.14 Connections to the PSTN

Large Office

Figure 1.14 Connections to the PSTN

Within the PSTN itself lies a network of networks, similar to the Internet, connecting offices from multiple telephony providers together into a massive, worldwide network. In order for all the telephony providers of the world to communicate together, a common signaling protocol must be used, similar to the way TCP/IP operates in the data realm. The voice signaling protocol used around the world is Signaling System 7 (SS7).

SS7 is an out-of-band (CCS-style) signaling method used to communicate call setup, routing, billing, and informational messages between telephone company COs around the world. When a user makes a call, the first CO to receive the call performs an SS7 lookup to locate the number. Once the destination is found, SS7 is responsible for routing the call through the voice network to the destination and providing all informational signaling (such as ring back) to the calling device.

Note SS7 is primarily a telephony service provider technology. You will not typically interface directly with the SS7 protocol from a telephony customer perspective.

PSTN Numbering Plans

Just as data networks use IP addressing to organize and locate resources, voice networks use a numbering plan to organize and locate telephones all around the world. Organizations managing their own, internal telephony systems can develop any internal number scheme that best fits the company needs (similar to private IP addressing). However, when connecting to the PSTN, you must use a valid, E.164 standard address for your telephone system. E.164 is an international numbering plan created by the International Telecommunication Union (ITU). Each number in the E.164 numbering plan contains the following components:

■ National destination code

■ Subscriber number

Note E.164 numbers are limited to a maximum length of 15 digits.

As an example, the North American Numbering Plan (NANP) uses the E.164 standard to break numbers down into the following components:

■ Central office or exchange code

For example, the NANP number 1-602-555-1212 breaks down as shown in Figure 1.15.

(E.164 Country Code) (E.164 National Destination Code) (E.164 Subscriber Number)

Figure 1.15 NANP Phone Number Example

(E.164 Country Code) (E.164 National Destination Code) (E.164 Subscriber Number)

Figure 1.15 NANP Phone Number Example

Even though the NANP defines specific categories of numbers that the E.164 standard does not include, the number still falls under the three broad categories, also shown in Figure 1.15.

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